1. Field of the Invention
The present invention relates to an improvement in a pedal reaction force applying apparatus for applying a pedal reaction force to a pedal member whose depressing stroke is electrically detected so that a hydraulically or otherwise operated brake is activated on the basis of the detected depressing stroke of the pedal member.
2. Discussion of Prior Art
There is proposed a brake-by-wire braking system, as a service braking system for a vehicle, in which a depressing stroke (pivot amount) of a pedal member is electrically detected so that an actuator such as a hydraulically operated device and an electrically operated motor is activated on the basis of the detected depressing stroke. In such a braking system, the pedal member receives only very small amount of pedal reaction force which is generated merely by a return spring provided in the pedal member. Therefore, there is a problem that the brake-by-wire braking system is difficult to operate, for a driver accustomed to a hydraulically or vacuum boosted braking system. In view of this, Patent Document 1 proposes a pedal reaction force applying apparatus which positively applies a pedal reaction force to a pedal member, by using a spring member such as torsion coil spring. Patent Document 2 proposes to displace a spring seat by rotating a cam with an electric motor, for changing a pedal reaction force depending upon a running condition such as snow-covered road surface. Patent Document 3 proposes to generate a braking force by using a depressing-force sensor and a stroke sensor, and control an amount of a pedal reaction force on the basis of difference between the generated braking force and its target value.
[Patent Document 1] JP-A-2001-239930
[Patent Document 2] JP-A-2001-247020
[Patent Document 3] JP-A-2001-278021
However, in the above-described conventional pedal reaction force applying apparatus, due to difference between characteristic of a pedal reaction force imparted by a spring member and characteristic of a pedal reaction force imparted by a conventional brake booster, a driver accustomed to a hydraulically or vacuum boosted braking system is likely to have a strange feeling in depressing the pedal member. In the technique of the Patent Document 1, the pedal reaction force is imparted merely by the spring member. In the technique of the Patent Document 2, the pedal reaction force is changed merely depending upon the road surface condition. In the technique of the Patent Document 3, the amount of the pedal reaction force is controlled merely on the basis of the difference between the generated braking force and its target value. That is, in any one of the conventional pedal reaction force applying apparatuses, the pedal reaction force in relation with the depressing stroke cannot be changed according to a desired change pattern so that it is not always possible to obtain a sufficiently satisfactory pedal maneuverability.
The present invention was made under the above-described background with object of providing a pedal reaction force applying apparatus in which it is possible to easily establish a pedal reaction force whose characteristic is close to the characteristic of a conventional brake booster, and accordingly easily obtain an excellent pedal maneuverability.